Analog-digital converter

ABSTRACT

A differential analog-digital converter is provided. The converter comprises a decision unit for evaluating a potential difference between two input signal lines, a number of charging units for each input signal line each configured to add a predetermined charge onto the respective input signal line, a number of discharging units for each input signal line each configured to remove a predetermined charge from the respective input signal line and a control unit for selectively switching each of the charging units and discharging units so that depending on one result of evaluating the potential difference between the two input signal lines one of the input signal lines is charged by adding the predetermined charge of the respective switched charging unit while the respective other of the input signal lines is discharged by removing the predetermined charge of the respective switched discharging unit.

PRIORITY CLAIM

The present application claims priority to the European Patentapplication identified as 11171761.7, filed on Jun. 28, 2011, andentitled “Analog-digital Converter,” the disclosure of which isincorporated by reference herein in its entirety.

FIELD

The present application relates to analog-digital converters, inparticular to successive approximation register (SAR) converters. Inparticular, the present application relates to an SAR converter topologyfor converting differential input signals.

BACKGROUND

SAR converters for analog-digital conversion are well known in the art.In particular, SAR converters are frequently used in integrated CMOSdevices since they provide a wide range of achievable resolution andconversion time and have competitive power efficiency. Furthermore, SARconverters can be implemented by optimally utilizing the advantages ofthe CMOS technology, which are small-sized switches and capacitorshaving well-defined relative capacitances. SAR converters can bedesigned for non-differential or differential input signals.

According to one well-known topology, differential SAR convertersinclude at least one capacitor bank on each input signal line. Eachcapacitor bank has capacitors of different values, usually with arelation between the capacitances by a factor of (n=integer). Eachcapacitor is connected between the signal line and a first referencepotential and is further associated with a switch, so that therespective capacitor can be disconnected from the first referencepotential and connected to a second reference potential. In the samplingphase, a voltage level of an input signal applied to the signal lines isstored on each capacitor of the capacitor bank. In a succeedingconversion phase, the second reference potential is appliedsuccessively, i.e. from the highest capacitance to the lowestcapacitance, on the capacitors while it is determined whether theresulting voltage on the input signal lines is positive or negative. Thedetermination results are stored as result bits in a register. Thecapacitor bank of the capacitor on which the second reference potentialis applied is selected depending on the result bit of the determinationjust carried out before.

According to another known topology, the capacitor banks of andifferential SAR analog-digital converters may have a C-2C designwherein the capacitor ladder is formed by switched capacitances C whichare coupled via a capacity with a capacitance of 2C. The differentialSAR analog-digital converter as described above has a capacitor bank oneach of the input signal lines, a decision latch to receive the inputsignal line voltage and a logic block which adjusts the switching of thecapacitor levels depending on a previous determination in the decisionlatch. Usually, the capacitors of the capacitor banks are all set to anegative reference potential for sampling the input signal and arepartially switched to a positive reference potential in the process ofconversion. When a capacitor is switched so that a positive referencepotential is applied, a capacitor charge depending on the switchedreference potential difference and the capacitance of the capacitor isloaded onto the respective input signal line, thereby increasing itspotential. Since in the conversion phase capacitors of both capacitorbanks on the input signal lines are switched the potentials of the inputsignal lines tend to be increased. The result is an increase of theinput common mode voltage on the input signal lines applied on thedecision latch. Since the input common mode voltage variation negativelyaffects the latch noise and the latch conversion speed, it is generallydesirable to keep the input common mode as constant as possible.

SUMMARY

In one embodiment, a differential analog-digital converter is provided.The converter comprises a decision unit for evaluating a potentialdifference between two input signal lines, a number of charging unitsfor each input signal line each configured to add a predetermined chargeonto the respective input signal line, a number of discharging units foreach input signal line each configured to remove a predetermined chargefrom the respective input signal line and a control unit for selectivelyswitching each of the charging units and discharging units so thatdepending on one result of evaluating the potential difference betweenthe two input signal lines one of the input signal lines is charged byadding the predetermined charge of the respective switched charging unitwhile the respective other of the input signal lines is discharged byremoving the predetermined charge of the respective switched dischargingunit.

The analog-digital converter provides charging and discharging units oneach input signal line. When after a decision step the voltagedifference between one input signal line and the other input signal lineneeds to be changed, this is done both by increasing the electricalcharge on one input signal line and by decreasing the electrical chargeon the other input signal line.

For example, after each decision step, two capacitors of two capacitorbanks on different input signal lines are switched, so that either thepotential of the one input signal line is increased and the potential ofthe second input signal line is decreased, or that the potential of thefirst input signal line is decreased and the potential of the secondinput signal line is increased.

Applying such a switching scheme may reduce or fully avoid variations ofthe input common mode voltage at the inputs of the decision latch. Theinput common mode voltage of the decision latch affects decisionaccuracy and comparison speed. In conventional designs, the input commonmode voltage variations have been traded for power by increasing thedimensions. By keeping the input common mode voltage variations low, thesizes of the electronic components of the decision latch can bedecreased and so chip area can be saved. Furthermore, smaller electroniccomponents allow for lower power consumption. Keeping the input commonmode voltage variations low also allows for an optimal common mode ofthe latch in terms of conversion speed.

Further the number of charging units and/or the number of dischargingunits may comprise switched capacitor units each having a capacitor anda semiconductor switch.

In particular, in at least one of the charging units and dischargingunits the respective capacitor is coupled with a first end at therespective input signal line and with a second end to the output of thesemiconductor switch.

It may be provided that in one setting step the predetermined charge toadd to the one input signal line may equal the predetermined charge toremove from the other input signal line.

To keep the input common mode voltage variation as constant as possible,the mean value of the potential on the input signal lines is to be keptconstant for each capacitor switching. In other words, the charges to beadded to the one input line equal the amount of charges which is removedfrom the other input signal line depending on the voltage difference tobe increased. Presuming a total overall capacitance of the input signallines with respect to a common ground potential to be equal, this leadsto no variation of the input common mode voltage at the input of thedecision latch. This scheme can be applied analogously in case thevoltage difference between the input signal lines shall be decreased.

It can be provided that the control unit is configured to perform anumber of decision steps by means of the decision unit wherein in onedecision step the predetermined charge to add to the one input signalline and the predetermined charge to remove from the other input signalline are adapted so that an absolute value of the potential change onthe two input signal lines is equal. In case the total overallcapacitance of the input signal lines with respect to a common groundpotential is not equal, it is necessary, to keep the input common modevoltage variation as constant as possible, that the charges to beapplied to the input signal lines results in the same potential changebut with different signs.

Moreover, the control unit may be configured to perform a number ofdecision steps by means of the decision unit wherein the predeterminedcharges to be added to the one input signal line and to be removed fromthe other input signal line within one decision step are decreasing fromone decision step to the next.

The control unit may be configured to perform a number of decision stepsby means of the decision unit wherein the predetermined charges to beadded to the one input signal line and to be removed from the otherinput signal line within one decision step are decreasing from onedecision step to the next by a factor of 2.

In another embodiment, a method for performing an analog digitalconversion is provided. The method comprises the steps of performing anumber of decision steps by evaluating a potential difference betweentwo input signal lines and by adapting the potentials of the two inputsignal lines, depending on one result of evaluating the potentialdifference between the two input signal lines both one of the inputsignal lines is charged by adding a first predetermined charge and theother of the input signal lines is discharged by removing a secondpredetermined charge.

It may further be provided that the first and the second predeterminedcharges are equal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a four-bit differential SAR analog-digitalconverter, according to an embodiment of the invention.

FIGS. 2A-2B show diagrams illustrating the latch noise and the latchspeed versus the input common mode voltage.

FIG. 3 shows a flow chart for illustrating the method for operating adifferential SAR analog-digital converter, according to an embodiment ofthe invention.

DETAILED DESCRIPTION

FIG. 1 shows a differential analog-digital converter 1 according to anembodiment. The analog-digital converter 1 has a topology generallyknown from SAR converters. The analog-digital converter 1 has a decisionlatch 2. The decision latch 2 can be a conventional operationalamplifier. The operational amplifier is configured to output an outputsignal the level of which indicates whether a differential input voltageapplied between a non-inverting input and an inverting input is positiveor negative. In the present case it is assumed that the decision latch 2outputs a signal low level, i.e. a logical “0” for a negative inputvoltage, and a signal high level, i.e. a logical “1” for a positiveinput voltage. The decision latch 2 serves for performing decision stepsand provides a decision output.

The non-inverting input of the decision latch 2 is connected with afirst input signal line 3 and the inverting input is connected with asecond input signal line 4. The first input signal line 3 is connectedto a first input terminal 5 via a first input switch 6 and the secondinput signal line 4 is connected to a second input terminal 7 via asecond input switch 8. The first and second input terminals 5, 7 are forreceiving a differential input signal U_(inp), to be digitally convertedby the analog-digital converter 1. The input switches 6, 8 can be commonCMOS switches or the like.

The first input signal line 3 is connected to a first capacitor bank 10and a second capacitor bank 11 and the second input signal line 4 isconnected to a third capacitor bank 12 and a fourth capacitor bank 13.In other words, each input signal line 3, 4 is connected to twocapacitor banks.

The capacitor banks 10, 11, 12, 13 in the illustrated embodiment eachhave three switched capacitor units 15. Each switched capacitor unit 15has one capacitor 16 the first end of which is coupled to the respectivesignal line 3, 4 and the second end of which is coupled to an output ofan inverter 17. The inverter 17 is capable of applying a first (lower)reference potential V_(ref−) or a second (upper) reference potentialV_(ref+) to the second end of the capacitor 16, depending on one ofcontrol signal d1 to d6 applied at a respective inverter input. Eachinverter 17 has a serially connected semiconductor switches preferablyCMOS transistors the gates of which the respective control signal d1 tod6 is applied while the series connection is coupled with the firstreference potential V_(ref−) and the second reference potentialV_(ref+). This allows to selectively apply one of the referencepotentials V_(ref+) and V_(ref−) to the second end of the respectivecapacitor 16.

In the present embodiment, the number of capacitors of each capacitorbank 10, 11, 12, 13 is 3; however, the number of capacitors of thecapacitor bank can differ and can be lower or higher than 3, dependingon the required resolution of the analog-digital converter 1.Furthermore, it is preferred that the capacitor banks 10, 11, 12, 13have the same number of capacitors; however, it is also possible to haveat least one capacitor bank the number of whose capacitors 16 differsfrom the number of capacitors of the other capacitor banks.

In particular, the control signals d1 to d3 are for controlling thereference potential applied on the second ends of the capacitors 16 ofthe first capacitor bank 10 and the fourth capacitor bank 13. Thecontrol signals d4 to d6 are for controlling the reference potentialapplied on the second ends of the capacitors 16 of the second and thethird capacitor bank 11, 12.

The three capacitors 16 of each of the capacitor banks 10, 11, 12, 13have capacitances of 4C, 2C and C to provide a binary conversion whilethe capacitor 16 having the capacitance of 4C provides for the mostsignificant bit and the capacitor 16 having the capacitance of Cprovides for the least significant bit.

The control signals d1 to d3 on the inverter inputs of the inverters 17of the first capacitor bank 10 and the fourth capacitor bank 13 areinverted. In the present embodiment the control signals applied to thefirst capacitor bank 10 are inverted control signals /d1 to /d3 and thecontrol signals applied to the first capacitor bank 10 are non-invertedcontrol signals d1 to d3.

According to the inverter design, a low level (logical “0”) of thenon-inverted control signals d1 to d6 lead to the output of the secondreference potential V_(ref+) to the second end of the respectivecapacitor 16 and the high level of the control signals dl to d6 resultsin applying the first reference potential V_(ref−) to the second end ofthe respective capacitor 16. Analogously, the inverted control signals/d1 to /d6 lead to the output of the first reference potential V_(ref−)to the second end of the respective capacitor 16 and the high level ofthe control signals d1 to d6 results in applying the second referencepotential V_(ref+) to the second end of the respective capacitor 16.

The control signals d1 to d6 are generated by means of a control unit20. Further, the control unit 20 controls the first and second inputswitches 6, 8. The main task of the control unit is to control theconversion process in its timely sequence such that a comparison stepand step of switching the switched capacitor units 15 are consecutivelycarried out.

Furthermore, a register 21 is coupled having register cells 22 to storecomparison outputs generated by the decision latch 2. The number ofregister cells 22 corresponds to the number of capacitors 16 in each ofthe capacitor banks 10, 11, 12, 13, i. e. four, and which is alsocontrolled by the control unit 20 to be operated as a shift register. Inother words, the control unit 20 controls the register 21 so that theresult of each comparison of a number of decision steps is successivelystored into the register cells 22 of the register 21.

In general, the operation of the control unit 20 serves for providingcontrol signals d1 to d6 that, depending on a previous result of adecision step performed by the decision latch 2, the loads of each inputsignal line 3, 4 is changed. This sequence is repeated for eachcapacitor 16 of each of the capacitor banks 10, 11, 12, 13, startingwith the capacitor 16 having the highest capacitance to the capacitor 16having the lowest capacitance in order of decreasing capacitances whilestoring the comparison outputs obtained in the decision latch 2 into arespective register cell 22 of the register 21 after each comparisonstep. As a general rule, the switching of the switched capacitor units15 is carried out such that the absolute value of the voltage differencebetween the charged input signal lines 3, 4 is reduced.

One aspect of the topology of the differential SAR analog-digitalconverter of FIG. 1 is to provide the possibility to either increase ordecrease the potential of each input signal line 3, 4 and that after atleast one decision step the potentials of both input signal lines 3, 4are changed at a time while the direction of the potential changes isdifferent. Therefore, two capacitor banks on each input signal line 3, 4are provided which are switched to keep the input common mode voltagevariation low.

In FIGS. 2 a and 2 b, it is shown diagrams depicting the latch noisesigma² determining the decision accuracy and the latch speed versus theinput common mode voltage of the decision latch 2. It can be seen thatlatch noise increases versus the input common mode voltage therebydecreasing the decision accuracy. Further, the decision speed isaffected by an increasing input common mode voltage.

To achieve lower input common mode variations, the control unit 20provides control signals d1 to d6, so that after each comparison stepcapacitors 16 are switched to reduce the absolute value of the potentialdifference between the input signal lines 3, 4. This is performed bychanging the potential of the first input signal line 3 towards thepotential of the second input signal line 4 and to change the potentialof the second input signal line 4 towards the potential of the firstinput signal line 3. The amount of the changes is determined by thevoltage difference between the first and the second reference potentialand the capacitance of the capacitor 16 to be switched.

In the following, in view of FIG. 3, a method for operating theanalog-digital converter 1 is described. In step S1 the first and secondinput switches 6, 8 are closed to pre-charge the first and second inputsignal lines 3, 4 with the input voltage U_(inp) applied at the firstand second input terminal 5, 7. During pre-charging, all control signalsd1 to d6 are reset to the low level (logical “0”) such that the secondends of the capacitors 16 of the first capacitor bank 10 are connectedto the first reference potential V_(ref−), the second ends of thecapacitors 16 of the second capacitor bank 11 are connected to thesecond reference potential V_(ref+), the second ends of the capacitors16 of the third capacitor bank 12 are connected to the first referencepotential V_(ref−) and the second ends of the capacitors 16 of thefourth capacitor bank 13 are connected to the second reference potentialV_(ref+).

After the first and the second input signal lines 3, 4 have beencharged, the first and second input switches 6, 8 are opened in step S2,so that the first and second input signal lines 3, 4 keep their chargedue to their attached capacities.

After the first and second input switches 6, 8 have been opened in stepS2, each capacitor 16 is charged with respect to the specific referencepotential applied to its second end. In step S3, a first comparison stepis performed using the decision latch 2, the decision result output D1of which is written into a first of the register cells 22 of theregister 21.

In step S4, depending on the first decision result D1, a first and afourth control signal d1, d4 is generated. Usually, each of the controlsignals d1 to d6 corresponds to one result obtained in the precedingconversion step. If in the present case a positive differential voltagebetween the first and second input lines 3, 4 exists, the first decisionstep would result in a high level result output (logical “1”). Thisresult leads to a change of the level of the fourth control signal d4while maintaining the other control signals at their set levels. If inthe present case a negative differential voltage between the first andsecond input lines 3, 4 is detected, the first decision step wouldresult in a low level result output (logical “0”). This result leads toa change of the level of the first control signal dl while maintainingthe other control signals at their set levels.

If the result of the first decision step is a logical “1”, the change ofthe fourth control signal d4 from a logical “0” to a logical “1” wouldlead to a change of the second reference potential V_(ref+) at thesecond end of the capacitor 16 having the highest capacitance C4 of thesecond capacitor bank 11 to the first reference potential V_(ref−).Simultaneously, it would lead a change of the reference potential at thesecond end of the capacitor 16 having the highest capacitance C4 of thethird capacitor bank 12 from a first reference potential V_(ref−) to ahigh reference potential V_(ref+.) Consequently, if a positivedifferential voltage is applied between the first and second inputsignal lines 3, 4 during the first decision step, this positive voltagedifference is reduced by decreasing the potential on the first inputsignal line 3 and by increasing the potential on the second input signalline 4.

Then, after the level of the first or fourth control signals are set anext decision step S5 is started and the decision result defines thesecond and fifth control signals d2 and d5 for the switched capacitorunit 15 related to the next highest capacitances in the same manner asdescribed above. In step S6 the switched capacitor units 15 are switcheddepending on the second or fifth control signal d2 and d5.

This sequence is carried out until in step S7 it is determined that thelast decision step has been carried out regarding switching a lowestavailable capacitance of the least significant switched capacitor unit15. In this case the conversion result is stored in the register 21 andcan be retrieved for further processing. If it is determined that therecent decision step did not refer to the switching of the lowestavailable capacitance of the least significant switched capacitor unit15, it is proceeded with step S4.

Substantially any adaptation of the potentials of the first and secondinput signal lines 3, 4, which leads to a reduction of the voltagedifference between the potentials on the first and second input signallines 3, 4, where one of the signal lines is increased in its potentialand the other of the first and second input signal lines 3, 4 isdecreased in its potential, the input common mode voltage variation iskept low. Preferably, the capacitors 16 of the capacitor banks 10, 11,12, 13 and the reference potentials V_(ref+), V_(ref−) are selected tomaintain the mean value of the potentials on the first and second inputsignal lines 3, 4 constant after each comparison step. This can beachieved, for example, by keeping the design of the input lines 3, 4 andof the capacitor banks 10, 11, 12, 13 substantially symmetrical, meaningthat the capacitances of the capacitors 16 of the first capacitor bank10 correspond to the capacitances of the capacitors 16 of the fourthcapacitor bank 13, respectively, while the capacitances of thecapacitors 16 of the second capacitor bank 11 substantially correspondto the capacitances of the capacitors 16 of the third capacitor bank 12,respectively.

1. A differential analog-digital converter comprising: a decision unitfor evaluating a potential difference between two input signal lines; anumber of charging units for each input signal line each configured toadd a respective predetermined charge onto the respective input signalline; a number of discharging units for each input signal line eachconfigured to remove a respective predetermined charge from therespective input signal line; and a control unit for selectivelyswitching each of the charging units and discharging units so thatdepending on one result of evaluating the potential difference betweenthe two input signal lines one of the input signal lines is charged byadding the predetermined charge of the respective switched charging unitwhile the respective other of the input signal lines is discharged byremoving the predetermined charge of the respective switched dischargingunit.
 2. The converter of claim 1, wherein the number of charging unitsand the number of discharging units comprise switched capacitor unitseach having a capacitor and a semiconductor switch.
 3. The converter ofclaim 2, wherein in at least one of the charging units and dischargingunits the respective capacitor is coupled with a first end at therespective input signal line and with a second end to the output of arespective semiconductor switch.
 4. The converter of claim 1, whereinthe predetermined charge to add to the one input signal line equals thepredetermined charge to remove from the other input signal line.
 5. Theconverter of claim 1, wherein the control unit is configured to performa number of decision steps by means of the decision unit wherein in onedecision step the predetermined charge to add to the one input signalline and the predetermined charge to remove from the other input signalline are adapted so that an absolute value of the potential change onthe two input signal lines is equal.
 6. The converter of claim 1,wherein the control unit is configured to perform a number of decisionsteps by means of the decision unit wherein the predetermined charges tobe added to the one input signal line and to be removed from the otherinput signal line within one decision step are decreasing from onedecision step to the next.
 7. The converter of claim 6, wherein thecontrol unit is configured to perform a number of decision steps bymeans of the decision unit wherein the predetermined charges to be addedto the one input signal line and to be removed from the other inputsignal line within one decision step are decreasing from one decisionstep to the next by a factor of 2.